Add metrics exporter to readme

Author: adinauer

sentry-opentelemetry/sentry-opentelemetry-agent/README.md

@@ -53,8 +53,16 @@ servers not being reachable in the logs.
 
 Example log message:
 ```
+ERROR io.opentelemetry.exporter.internal.grpc.OkHttpGrpcExporter - Failed to export spans. The request could not be executed. Full error message: Failed to connect to localhost/[0:0:0:0:0:0:0:1]:4317
 ERROR io.opentelemetry.exporter.internal.grpc.OkHttpGrpcExporter - Failed to export metrics. The request could not be executed. Full error message: Failed to connect to localhost/[0:0:0:0:0:0:0:1]:4317
 ```
 
+### Traces
+
 To turn off exporting of traces you can set `OTEL_TRACES_EXPORTER=none`
 see [OpenTelemetry GitHub](https://github.com/open-telemetry/opentelemetry-java/tree/main/sdk-extensions/autoconfigure#otlp-exporter-span-metric-and-log-exporters)
+
+### Metrics
+
+To turn off exporting of metrics you can set `OTEL_METRICS_EXPORTER=none`
+see [OpenTelemetry GitHub](https://github.com/open-telemetry/opentelemetry-java/tree/main/sdk-extensions/autoconfigure#otlp-exporter-span-metric-and-log-exporters)